To study the effect of glutathione (GSH) on expression of CD4 human peripheral blood lymphocytes(PBL). Human PBL were cultured in vitro for 72 h with or without GSH or glutithione nonoethyl ester (GSET), an analog of GSH known to cause marked increase in lymphocyte GSH level. The PBL were washed, treated with anti-CD4 murine monoclonal antibodies (OKT4) and fluorescein isothiocyanate-labeled goat anti-mouse IgG. The cells were then analyzed on a FACStar Plus (Becton Dickinson) flow cytometer. The frequency and intensity of the CD4+ cells cultured with and without GSH or GSET were compared. The cellular GSH content of the PBL was analyzed according to griffith (anal. Biochem. 106, 207). Elevation of cellular GSH increases the population of CD4+ cells.